The harder material wears the softer material away. (hopefully the rotor wears the pad). Pads that function primarily by abrasion have a higher wear rate and tend to fade at high temperatures. When these pads reach their effective temperature limit ,they will transfer pad material onto the rotor face in a random and uneven pattern.It is this "pick up" on the rotor face that will cause the thickness variation and roughness.(vibration felt when braking)
With adherent friction ,some of the pad material diffuses across the interface between the pad and the rotor and forms a very thin , uniform layer of pad material on the surface of the rotor.As the friction surfaces of both rotor and pad then comprised basically of the same material . the material can now cross the interface in both directions and the bonds break and reform.In fact , with adherent friction between pad and rotor,the bonds between pad material and the deposits on the rotor are transient in nature-they are continually being broken and some of them continually reforming.
There are no pure abrasive or pure adherent friction materials.With many contemporary pad formulas,the material must be abrasive enough to keep the the rotor surface smooth and clean.Organic and semi metallic pads of the past were more abrasive than adherent and were severely temperature limited.All of the current generation of "metallic carbon" , racing pads utilize mainly adherent technology as do many performance pads and they are temperature stable over a much higher range.
Bedding (run in) New brake pads and rotors need to go through a bedding process before they can perform reliably and efficiently under hard driving conditions.Therefore there are two bedding process : bedding of the friction and bedding of the rotors.It is best to bed new pads on bedded rotors , that are smooth and flat with no hot spots or bluing.All friction materials contain volatile elements used as binders. In the initial thermal cycling of the material these volatile s must be burnt off slowly to avoid both fade and uneven deposits or will otherwise farm a gaseous layer between the friction material and the rotor inter face during hard breaking The process seeks to gradually increase the temperature in the components without inducing thermal shock.
A bedded pad will exhibit a layer of discoloured material from 1.5 to 3 mm thick.
Bedding also thermally relaxes the rotor, removes any burrs or machining oils from the rotor, and wipes a layer of pad material onto the rotor surface creating a good marriage between the rotor band brake pads.This will prevent thermal shock, distortion and the formation of hot spots and ensure maximum disk life.
Installation of new rotors usually involves washing with soap and water or using one of the proprietary brake clean compounds.
The bedding procedure consists of several stops of increasing severity with a brief cooling period between them until the entire surface of the rotor is evenly discoloured. The sequence of stops is designed to ensure that the rotor is warmed slowly and reaches a temperature consistent with its final use. After the last stop, the system should be allowed to cool to ambient temperatures. Typically, a series of ten increasingly hard stops from 96kmh to 8 kmh with normal accelaration in between should get the job done for a high performance street pad.During pad or rotor break in , you must not come to a complete stop-possible non uniform pad material transfer or pad impairing.
You will begin to smell the pads at the 5th or 7th stop and the smell should diminish before the last stop.A powdery gray area will become visible on the edge of the pad where the paint and resins of the pad are burning off. When the gray area on the edges of the pads is about 1/8" deep . the pad is bedded.For a race pad, you should typically run four 130 kmh to 8 kmh and two 160 kmh to 8 kmh.When bedding it will be necessary to raise the system temperatures during break in to the range that the pad material can establish its layer completely and uniformly on the rotor surface.Following this procedure will maximize pad life,maximize the co efficient of friction between the materials.Perform trial brake applications at low speeds keeping in mind that brake effectiveness may be somewhat reduced during the first brake application.Do not excessively force or perform continuous brake application in order to achieve faster bedding in.
With abrasive pads the rotor can be considered bedded when the friction surfaces have attained an even blue colour.With the carbon metallic type pads.bedding is complete when the friction surfaces of the rotor are a consistent gray or black.In any case,the discoloration of a completely broken rotor well be completed and uniformed.
When the brake system is bedded in for the first time, a slight increase in pedal travel and travel effort may occur.After the first fade and proper cooling,the system will maintain its optimum performance at all temperatures.If vibration is just felt,chances are the temperature has never reached the point where cementite begins to form.In this case simply fit a good set of "semi metallic" pads and using them hard may well remove the deposits and restores the system to normal operation.If only a small amount of material has been transferred i e vibration is evident,send the rotors for skimming.You also have to bed new pads on used rotors and likewise new rotors with used pads.
Brake pads are wear and tear components that must be checked at regular intervals.If the thickness of friction material remaining on a pad is less than 2mm or the wear indicator releases ,replace all pads.Some cars are fitted with an electronic pad sensor to warn the driver that the pads are worn.Others may be fitted with a protruding metal pin that will scrape the surface of the disc rotor making a final sound.Check your pads after every oil change by having them blow cleaned.Brake pads on automatic transmission cars usually wear faster than manuals.Brake pads must be changed in pairs.
Finally observe the breaking in process and never leave your brake pedal floored after you have used the brakes hard.
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